A typical toilet used in domestic applications includes a toilet bowl mounted on a floor surface and in communication with a drain to take away the contents of the toilet bowl, and a water supply tank at a higher elevation that provides the proper amount of water during a flush cycle of the toilet bowl. In order to re-fill the tank after a flush cycle, a float in the toilet tank moves down during a flush cycle and opens a fill valve to supply replacement water in the tank. The float responds to the rising level of the liquid in the tank to close the fill valve. If the drain opening of the toilet is clogged and the toilet is flushed, the fresh replacement water coming from the toilet tank to the bowl has no escape, the water level in the toilet bowl rises, and there is a hazard of overflow of the contents in the toilet bowl.